Experimental study on flame characteristics of propane fire array

This paper presents an experimental analysis on the flame characteristics of multiple fires burning by tests of square fire array consisting of even number of equidistant burners, using propane as the fuel. We propose to use the temperature along the centerline of fire array, instead of visual observations, to identify flame merging and examine the characteristics of flame structure under different states of flame merging. It is revealed that with decreasing flame spacing, the flames undergo a physical transition in the centerline temperature of the fire array, induced by the formation of the fuel-rich region in the lower region above the burner surface. Analyses on fuel-rich region and axial temperature, in a quantitative way, provide physical details of the changes of flame structure when the flames that burn on discrete fuel beds undergo physical transition to become one single flame with decreasing flame spacing. It is inferred from the temperature data that the air entrainment is more restricted with increasing merging level of flames. A new method in terms of the centerline temperature of fire array is proposed to determine the critical heat release rate for the onset of flame merging. It is found that the heat release rate Q˙D∗, using the fire array size D as the characteristic length scale, can be used to correlate the flame height of a merged fire array. It is further indicated that a general linear form L/D=3.7Q˙D∗2/5−c can be used as a general correlation of flame height for fire arrays in different merging levels, in which the parameter c characterizes the deviation from the Heskestad curve under different flame spacings. For non-merged fire array, the flame heights of separate flames mainly depend on the heat release rate, while the flame spacing and the number of flames have minor effects.